1. Field of the Invention
The present invention relates to a free-space optical communication system in which an optical beam transmitted in free space is used to perform wireless communication between a free-space optical communication apparatus provided at one point and other free-space optical communication apparatuses provided at a plurality of points.
2. Description of the Related Art
The system shown in FIG. 5 has been proposed as the above type of free-space optical communication system that uses an optical signal to perform wireless communication. The system in FIG. 5 performs optical communication between a central base 50 provided at a point and individual communication apparatuses 51a to 51c provided at a plurality of remote points.
Referring to FIG. 5, an optical signal emitted from a light source 52 in the central base 50 is converted into a slightly broadened parallel optical beam 54 by an optical system 53. In the central base 50, the rotational position of a movable mirror 55 is set at an angle that enables optical beam transmission to one of the communication apparatuses 51a to 51c, and optical beam transmission is performed by using the movable mirror 55 to reflect the optical beam to that apparatus.
This system sequentially scans the optical beam over the communication apparatuses 51a to 51c for communication by changing the angle of the movable mirror 55 in a predetermined order and in accordance with predetermined angles in such a way that, upon termination of transmission to the communication apparatus 51a, the mirror 55 is driven to an angle for transmission to the communication apparatus 51b and, upon termination of transmission to the communication apparatus 51b, the mirror 55 is driven to an angle for transmission to the communication apparatus 51c. 
The scanning is performed at high speed so that the user of each communication apparatus is not aware of any delay, particularly when the apparatus receives a signal.
In addition, the optical signal can carry data at high speed because it has no frequency range limitation, as in radio waves. Thus, the optical signal enables communication with sufficient data capacity, even in intermittent transmission using sequential scanning.
Japanese Patent Laid-Open No. 2000-224112 proposes a base apparatus which includes a light source and an optical detector and which uses a mirror to scan each communication apparatus and which performs bidirectional communication with the communication apparatus.
Also, in Japanese Patent Laid-Open No. 2000-224112, in order to prevent the optical beam from being deflected by a change in the posture and angle of the base apparatus as a result of base or building deformation caused by wind, vibrations, or temperature changes, optical-beam-direction correcting functions have also been proposed. In these functions, an accelerometer and a temperature sensor are used for correction, and five arranged photodiodes are used to correct the direction of the optical beam from the output of each photodiode.
In order to increase the reliability of free-space optical communication as described above, it is preferable for the base apparatus to have an automatic beam-tracking function in which, if the base or the building is inclined, the mirror angle is feedback-controlled so that the optical beam can be emitted towards the communication apparatus by always detecting the direction of the communication apparatus.
Among the above systems of the related art, the optical-beam-direction correcting system including an accelerometer and a temperature sensor does not directly correct the direction of the emerging optical beam by performing automatic tracking. Accordingly, the relationship between the output of the accelerometer or temperature sensor and correction in the optical beam direction differs depending on certain conditions such as the location, the base, and the building structure. Thus, it is impossible to ensure the correction.
Among the above systems of the related art, the optical-beam-direction correcting system including five arranged photodiodes has a problem in that, when scanning the communication apparatuses, it is impossible to perform the scanning at high speed, and therefore each communication apparatus is prevented from being aware of a delay because a long time is required to perform the steps of directing the mirror to a specified communication apparatus, reading the outputs of the five photodiodes, performing arithmetic operations, and correcting the optical beam direction by driving the mirror again.